I have had them slide before, I think they screw has a slight tip that wears down..YOu can buy replacements through amazon I believeThe weird thing is the Intermatic 104P timer - the ON stop fell off. I found it loose a month ago and tightened it by hand. Today it was missing and I found it on the floor. I put it back on and used a pliers to tighten it. Anyone else have that problem?